What’s In a Timber Cruise?

Photo 1- Forestry tools used to cruise timber. This photo includes a Rite in the Rain field book, handheld GPS, Cruz All basal area tool, Relaskop, laser, compass, diameter tape, clinometer, increment borer, and flagging (Photo: Rachel Mazzacavllo, WA DNR)

By Rachel Mazzacavallo, WA DNR Service Forestry Coordinator
Rachel.mazzacavallo@dnr.wa.gov

So, what is a timber cruise anyway? Many forest landowners understand why one might have a timber cruise performed, but the intricacies of what goes into creating one of these and how it can differ from other forest inventories is less universally understood.

Overview
A timber cruise, simply, is an assessment of merchantable timber. It usually differs from other forest inventories in that a volume estimate is one of the primary products. A timber cruise typically consists of a random "sample" or measurement of a set of plots based on a predetermined cruise design. The design dictates the plot intensity, plot grid, and appropriate plot type. The sample is used to develop a statistically significant estimate of merchantable timber products in a given area. For cruises aimed at establishing a property valuation, such as an estate cruise or appraisal, the entire parcel or ownership would be within the cruise unit. For a timber harvest, the cruise area would be the areas that will have timber cut and removed- including road rights-of-way- but would not include protected areas that are excluded from active timber harvest such as stream buffers or unstable slopes. The unit is defined by the purpose of the cruise. Other than to develop a property valuation or to harvest timber, a landowner may need a timber cruise to grant an easement (road right of way, conservation easement, etc), or to account for timber theft. For activities outside of these, including long-term forest management planning, a more basic inventory would likely be appropriate.

Components of a timber cruise
Design. The first component of a timber cruise is the cruise design. Every cruise requires a robust design to ensure that the objects are met and that estimates can be made with the appropriate level of confidence in the data. For most timber sales, cruise plots are distributed throughout the sale area and then extrapolated to make stand-level estimates. In some less common cases, such as in small areas or with high-value or specialty products, all the trees are included in the measurement. The number of sample plots is based on the stand acreage, the intended use of the cruise, and how uniform the stand is. The more variable, or heterogenous, a timber stand is the more plots that will need to be installed. The more uniform, or homogenous, a stand is the fewer plots that are necessary to obtain a high degree of confidence in the cruise. Ideally, each plot will pick up 5-10 measurement trees.

Plots can be measured using either fixed radius or variable radius. Both plot types can produce valuable results, so it is up to the cruiser to determine which type is most appropriate for each timber cruise.

Data Collection. Once a plot is established and all the "in" trees are identified the cruiser will record several key tree measurements. Tree species, such as Douglas fir or ponderosa pine is recorded. The tree diameter is taken at breast height with a diameter tape. Most forester tapes have one side that indicates distance and one that indicated diameter from a circumference measurement. The taper, or the reduction in diameter as you look up the main stem of the tree is recorded. Since trees are cone shaped we need to use the taper to accurately estimate volume. Heights are measured and can be taken numerous ways and used to calculate the volume. Common height measurements include to the top of the tree, to a specific diameter point on the tree (for example, at 5 inches in diameter the tree is 80 feet tall), or to a major fork. The cruiser will then determine how the tree will be utilized, or segmented, during harvest and records the sort and grade of each long segment that can be manufactured from the tree. This information determines the kind of product that can be made from the tree. Defect along the main stem of the tree is noted and used to make volume deductions from each log segment. Other metrics such as live crown ratio, leave, or cut may also be recorded.

Photo 2- Part of a timber harvest unit summary from the cruise program Superace. The columns represent unit estimates of average trees per acre, basal area, and volume by the species. From left to right: tree species, sample trees (number of trees measured in the unit by species), Trees/ac (estimated average number of trees per acre by species), AVG DBH (Average diameter of the sample trees), bole length (the length of the tree with merchantable timber), relative density (metric of tree density and competition), Basal area (average square footage occupied by tree stems per acre), and the rest are volume measurements.

Data Synthesis and Statistics. Typically, once a timber cruise is complete, all the plot data is entered into a cruise program or database. Individual tree data is combined to produce stand summaries. Basal area (BA), trees per acre (TPA), and average diameter by species are common timber cruise metrics in addition to volume. Volume is commonly reported categorized by species, sort, and grade. Volume is expressed in board feet, or 1 inch x 12 inches x 12 inches, and is typically reported by the thousand board feet denoted as MBF or reported as board foot per acre denoted as BF/AC. Volumes can also be calculated manually using the Scribner Decimal C log scaling rules.

Population sampling, which most timber cruises are, use statistics to determine how reliable, or how much you can trust, that the sample accurately represents the population. What is the likelihood that the trees that were measured represent the entire forest? Cruisers don’t just hope the plots represent the forest; they use statistics to ensure that they do. Important statistics tell the cruiser information such as averages (BA, TPA), and the standard deviation (how variable the unit is). Error statistics tell the cruiser how likely this cruise will predict the actual stand when repeated numerous times. It’s no surprise, but most cruises are designed to produce a fairly small sample error.

Want to learn more? Pick up a copy of Log Scaling and Timber Cruising (Bell, revised 2007)

How to have your forest cruised
The best way to obtain a timber cruise of your woodland is to hire a professional consulting forester. These individuals differ from government assistance foresters because they will cruise and manage the harvest and marketing of your timber. They will represent the landowner in negotiations and make decisions based on your goals and best interests. They work for and are hired directly by the landowner. Talk to neighbors or small forest landowner groups who have worked with consulting foresters to get several recommendations for foresters to hire.

Rachel Mazzacavallo, WA DNR Service Forestry Coordinator

Rachel.Mazzacavallo@dnr.wa.gov

(509) 774-7170

Edited by Kelsey Ketcheson